Electronic equipment, particularly computer equipment, frequently depends on clock pulses of a stable frequency to control various functions. However, with increasing component density and multiple functions on one board there is a need for multiple clock frequencies. For example, new hard and floppy disk drive controllers with serial and parallel I/O ports on one board for use with a microcomputer may require five or more different clock frequencies, none of which may be available from the central processing unit or its peripherals.
Multiple clocks with different frequencies must normally be generated by different oscillators, each with a crystal as a reference. Crystal oscillators are expensive and large, and therefore, consume large amounts of board area. Phase lock loops can also generate multiple clocks of different frequencies from a single reference source such as a crystal oscillator and a voltage controlled oscillator (VCO). Although these circuits are also generally quite large, they can be formed and reduced in size in an integrated circuit (IC) with other circuity. However, typical VCOs require a control circuit with pads to external resistors, capacitors, individual oscillator components, and a reference oscillator. Such control circuitry and pads require space, as do the external components. In addition, VCOs generate frequency modulated noise which can disturb other computer functions and contribute to electromagnetic interference (EMI) problems.
Control circuits are also known which compare the frequency of a VCO to a desired frequency and transmit the difference in digital form to a digital to analog converter to control the VCO. However, these circuits generally are not flexible and require a large number of parts for a single VCO. Other clock circuits are known which are generally inflexible.